roll (expr1, expr2)

1. expr1:

   Symbol or stream of data to be rolled with an adjusted expiration date.

2. expr2:

   Root of the commodity which will be used to determine expiration date.  Insert a number to roll the expiration date that many days early. 

   examples

   roll("/GCL<0>", "1tBRN")
   
   roll("/GCL<0>", 1)
   
   "1tBRN<0>" - roll("/GCL<0>", "1tBRN")
   
   2*uc(roll("/GRB<0>","/CL"),bbl)+uc(roll("/GHO<0>","/CL"),bbl)-3*"/GCL<0>"